# ---
# title: 376. Wiggle Subsequence
# id: problem376
# author: Indigo
# date: 2020-12-12
# difficulty: Medium
# categories: Dynamic Programming, Greedy
# link: <https://leetcode.com/problems/wiggle-subsequence/description/>
# hidden: true
# ---
# 
# A sequence of numbers is called a **wiggle sequence** if the differences
# between successive numbers strictly alternate between positive and negative.
# The first difference (if one exists) may be either positive or negative. A
# sequence with fewer than two elements is trivially a wiggle sequence.
# 
# For example, `[1,7,4,9,2,5]` is a wiggle sequence because the differences
# `(6,-3,5,-7,3)` are alternately positive and negative. In contrast,
# `[1,4,7,2,5]` and `[1,7,4,5,5]` are not wiggle sequences, the first because
# its first two differences are positive and the second because its last
# difference is zero.
# 
# Given a sequence of integers, return the length of the longest subsequence
# that is a wiggle sequence. A subsequence is obtained by deleting some number
# of elements (eventually, also zero) from the original sequence, leaving the
# remaining elements in their original order.
# 
# **Example 1:**
# 
#     
#     
#     Input: [1,7,4,9,2,5]
#     Output: 6
#     Explanation: The entire sequence is a wiggle sequence.
# 
# **Example 2:**
# 
#     
#     
#     Input: [1,17,5,10,13,15,10,5,16,8]
#     Output: 7
#     Explanation: There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].
# 
# **Example 3:**
# 
#     
#     
#     Input: [1,2,3,4,5,6,7,8,9]
#     Output: 2
# 
# **Follow up:**  
# Can you do it in O( _n_ ) time?
# 
# 
## @lc code=start
using LeetCode
function wiggle_max_length(nums::Vector{Int})::Int
    len = length(nums)
    (len < 2) && return len

    pre_diff = nums[2] - nums[1]
    res = (pre_diff == 0 ? 1 : 2)
    for i in 3:len
        diff = nums[i] - nums[i - 1]
        if diff > 0 && pre_diff <= 0 || diff < 0 && pre_diff > 0
            res += 1
            pre_diff = diff
        end
    end
    return res
end
## @lc code=end
